In nearly all photography, the goal is to produce clear, sharp images of a subject. This is typically achieved by focusing the camera's lens. Thus, most lenses used on cameras incorporate some arrangement to allow for adjustment of the lens so subjects will be in focus over a range of distances. In recent years a variety of autofocus systems have been developed. These systems typically include a device for measuring the distance to the subject and using this measurement to actuate a motor or solenoid which adjusts the lens bringing the object into focus. Devices used to measure distance in such systems include ultrasonic, or infrared range finders. Alternatively, passive systems incorporating a moving mirror and a pair of photodiode arrays are commonly used. These systems adjust the focus distance until contrast is maximized without ever needing to know what the actual distance is.
Under most conditions, autofocus systems of the type described above work well when objects to be photographed are at rest. Passive systems are less effective when the subject is not well lighted. Ultrasonic systems are ineffective when photographing a subject through glass. Infrared systems can be fooled by unusual infrared reflectance from the subject. All such systems are less effective when photographing moving objects. There are delays in bringing the moving object into focus. Also, many autofocus systems disable the shutter. If the subject has moved, the desired image is lost before the shutter is enabled. This is particularly troublesome when photographing small, active children. All of these problems are exacerbated when the camera is equipped with a zoom lens. There is a real need to overcome the problems associated with autofocus, particularly if the camera is to be used in school photography where large numbers of active children must effectively and efficiently be photographed.
Given the problems associated with autofocus, school photographers have in the past typically used fixed focus cameras designed for portraiture. These cameras are generally limited to this function exclusively. Since the focus of the lens is not adjusted for each subject, either the subject or the camera must be moved to achieve proper focus. Manual measuring techniques are used to measure distance. For example, a string can be attached to the camera. The string has a marker indicating the proper focus distance. Checking of focus distance is accomplished by holding the string near the subject. This creates several potential problems. First, this method requires that the photographer reach near or even touch the subject. This invasion of the subject's personal space may make the subject very uncomfortable. Second, after the photographer measures the distance the photographer returns to the camera. Unbeknownst to the photographer, the subject may move out of focus after the measurement, but before the photograph is taken.